US1858838A - Cable sheath bonding - Google Patents

Description

May 17, 1932. K. w. MILLER ET AL CABLE SHEATH BONDING Filed MELICh 28, 1930 Z3 ZZ f e e Z//a er.

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Patented May 17, 1932 Unir-sn STATES PATENT orties KENNETH W. MILLER AND DENNEY W. KOPER, OF CHICAGO, ILLINOIS CABLE SHEATH BONDING Application led March 28, 1930.- Serial No. 439,594.

rlhis invention relates to sheath bonding lengths are crossbonded or connected serially and more particularly to the bonding toin transposed relation by means of bond gether of the metallic sheaths of single-conwires and at every cross-bond location one of ductor underground cables employed in the cross bonds is connected to a ground. To

three-phase transmission. Secure the benefits of this scheme of connec- 55 Vhen alternating currents are delivered tion, the transpositions must be connected in over the conductors of single-conductor the same phase sequence and the ground concables enclosed in separate metallic sheaths, nection be similarly connected at every bond there is induced in such cable sheath a voltage location.

which is directly proportional to the con- The reduction of voltage between cableGO ductor current and to the length of each secsheaths is accomplished in our invention tion of cable between manholes. if the without the use of special sheath bonding desheaths of each of the cables are joined by vices and this is an important feature of our means of ordinary joint sleeves, and are invention.

solidly bonded together in each manhole, The reduction of voltages between cable 65 electrical continuity of the sheath circuit is sheaths is very important since it has been preserved, thus providing a closed electrical found in iield practice that the possibility ot' circuit. sheath corrosion due to A. C. voltages inn such case, the induced sheath voltages creases very rapidly with the voltages because large sheath currents to flow resulting tween cable sheaths. Our invention accom-' 7o in large heat losses which considerably reduce plishes this voltage reduction without the the current capacity of the cable and conserequirement of any special apparatus other quently add to operating cost. It, however, than a ground in every manhole or an auX- the sections of the sheaths are insulated from iliary ground cable. In ordinary undereach other and are thereby made discontinuground construction, other cables are usually 75 ous by means of the interposition of insulatpresent in the same conduits and the sheaths ing sleeves or joints, potentials will be estabof such cables, which are normally electricallished between the sections and between a secly continuous and solidly bonded and tion and ground. By specially connecting grounded, may be employed as the auxiliary the sheaths of the sections by means of bond ground cable. 80 wires or impedances, sheathcurrents may be In one form of cross-bonding well known prevented cr greatly restricted and sheath in the art in which the three cable sheaths losses practically eliminated. In this case, are all solidly bonded together and grounded the induced sheath voltages are not consumed in every third manhole, we have found that in producing sheath currents and are therethe voltages existing between sheaths are S5 :tore present on the sheath. Various types of normally two-thirds greater than those eX- suitable insulating sleeves have been devised isting between sheaths for the form of Crossor interrupting the electrical continuity ot bonding in accordance with our invention.

the sections of the sheath of such cables and This important feature of our invention is several bonding methods have been devised due to the fact that in our method of bond- 90 for connecting the sections of the cable ing the voltages between sheaths are the sheaths to prevent sheath losses. same as if the sections of the sheaths were rlhe present invention introduces a new connected in delta. This is a distinct admethod of connecting the sections and vantage over other forms of bonding well sheaths or" single conductor cables which inknown to the art, in which the voltage be- 95 cludes the desirable features of a continuous tween sheaths is equivalent to that which metallic path along the sheath circuit and would result from the star connection of the negligible sheath losses. sections of the sheaths.

Briefly stated, to prevent the accumulation fxith the above in view the principal obof sheath voltage along the line, the sheath jects of the present invention are: to reduce 10 induced sheath currents and losses, thereby increasing line rating and reducing operating costs; to avoid accumulative effect of induced sheath voltages along the line; to atl'ord electrical continuity of the sheath circuit, thus providing a return path for fault currents; to hold sheath voltages between cable sheaths to a minimum value consistent with the above aims, thereby reducing the possibility of sheath corrosion to a ininimum; and to accomplish the above objects at a minimum cost employing a minimum amount of special equipment.

Other and further objects will appear from the following detailed description of a preferred embodiment of our invention illustrated in the accompanying drawings in which:

Figure 1 is a diagrammatic or schematic view showing a portion of a threephase cable system bonded in accordance with the present invention;

'Figures 2, 3 and 4 are vectorial voltage diagrams illustrating the sheath voltage relations of the system illustrated in Figure 1;

Figure 5 is a vector voltage diagram representing voltages which result with sheaths cross-bonded according to a method already known to the art and in which the three cable sheaths are all solidly bonded together and grounded in every third manhole;

Figure 6 is a schematic wiring diagram for terminating bonding according to our invention at the end of a three-phase transmission line or at a solidly grounded point; and

Figure 7 is a schematic wiring diagram for an alternative method of terminating bonding at the end of such a line or at a solidly grounded point.

Figure 8 is a diagrammatic or schematic view showing a single phase cable bonded in accordance with the present invention.

Figure 9 is a schematic wiring diagram illustrating an alternative connection of the ground cable in a three phase cable system bonded in accordance with my invention.

Figure 10 is a vector voltage diagram i'ls lustrating the sheath voltage relations of the system illustrated in Figure 9.

In Figure 1 of the drawings, we have schematically illustrated a three-phase cable system comprising three single conductor cables, 10, 11 and 12 comprising conductors 13, 14 and 15 respectively, each covered with a suitable insulation in turn surrounded by metallic sheaths 16, 17 and 18 respectively. The electrical continuity of these sheaths is interrupted in substantially equal lengths by any suitable insulating sleeve or joint schematically illustrated in Figure 1 by gaps 19 between the sections or lengths of each of the cable sheaths.

In order to simplify the disclosure of the invention, the sheath sections in Figure 1 are denoted by Roman numerals I to V serially, and the phases are identified by letters A, B and C.

The ordinary practice in this country is to install cable in conduits with manholes at suitable intervals. One manhole is schematically represented by the dotted rectangle 26 in Figure 1. The insulating joints 19 in the lead sheaths of the cables are usually installed in these manholes and the sheath bonding connections 21, 22, ground 24 (and bonding devices when used) are most conveniently installed in the manholes.

To prevent the accumulation of sheath voltage along the line, the sheath lengths are cross-bonded or serially connected in transposed relation by means of jumpers or bond wires 20. As will be seen from Figure 1, the sections of the sheaths are connected by bond wires 2O and 21 in the following order: IA, IIB, IIIC, IVA, VB.

Further examination of Figure 1 will show that there are two other similar circuits of the sheath sections, one starting with sec tion IB and the other starting with IC. Further, it will be seen that only one of these three sheath circuits is grounded in any one manhole by means of a jumper or wire 22 connected between the cross-bond 21 and a suitable ground 24, or to an auxiliary' ground' cable 23 which is preferably grounded at one or more points 24. Each of these three sheath circuits is grounded in turn in connective manholes.

It will be apparent to one skilled in the art that as we proceed progressively from left to right in Figure 1, there are three equivalent sheath circuits with one ground at each section point and with any one circuit groundsimilar triangles, that the voltages between sheaths are similar in every manhole, that the maximum voltages that can occur between sheaths occur in the manholes and that this maximum voltage cannot exceed the voltage induced in one section length of the sheath.

The three sheath voltages in the manhole 26 between sections III and IV of Figure 1, for example, are denoted by the distance between ground and the letters X in Figures 2, 3 and 4.

In order to more clearly emphasize the contrast between the voltage reduction between cable sheaths accomplished by our invention and that accomplished by a form of crossbonding well known to the art in which the i3 three cable sheaths are all solidly bonded together and grounded in every third manhole, reference may be had to Figure 5 of the drawings in which we have shown by vectorial representation the voltages between cable sheaths forthis method. The three voltages between sheaths in two-thirds of the manholes in this case are the values between the letters X Figure 5. In actual value these voltages between cable sheaths in either of the two intermediate manholes are approximately the square root of three times the 'voltages induced in one sheath section between insulating sleeves.

This reduction of voltage between cable sheaths is accomplishedfin our invention without the use of special sheath bonding devices since ordinary wire is sufficient to furnish the desired bond. Qur invention further accomplishes this voltage reduction -without the requirement of any special apparatus other than a ground in every manhole or an auxiliary ground cable which is usually present in ordinary underground construction, such cables being normally electrically continuous and solidly bonded and grounded. Ve have found that our cross-bonding arrangement furthermore materially reduces the voltages between sheaths below that which would result by using any other form of sheath bonding at present known inthe art and in which special bonding devices are not employed. In addition, it is to be observed that the sheath circuit in our cross-bonding arrangement is continuous for the three phases of the single-conductor cable line as a whole and, therefore, interruptions of short lengths in the auxiliary cable are permissible without adversely affecting the return path for failure currents in the sheath bonding connection of our invention. This is equally true as regards the omission of one or more ground points 24 where an auxiliary cable is not employed.

In Figures 6 and 7, we have illustrated two of the many possible methods of beginning or terminating sheath bonding in accordance with our invention. It is to be noted that in these figures the sections of cable sheath, the bond wires, and the auxiliary cable or equivalent grounds are all shown as single lines, these representations being merely schematic wiring diagrams the interpretation of which will be obvious to those skilled in the art.

In Figure 6, one single-phase impedance bonding device 25 is employed in each of the rst .two manholes adjacent to the solidly bonded terminal point for connecting the sheaths of the initial section of phase C with the sheath of the succeeding section of phase A. These impedance bonding devices are not essential and may be omitted if desired, their chief function being merely to act as an additional guarantee of the electrical continuity of the sheath circuit for returning line failure current.

In Figure 7 we have illustrated another method of beginning or terminating sheath bonding in accordance with our invention in which a special connection is necessary in only one manhole adjacent to the end of the line that is adjacent to the solidly bonded terminal point. Thus the sheaths of the end section of the three conductors are shown as not interconnected or not bridged except for one cross-bond 20. As in the case of the arrangement shown in Figure 6, the open ends of sheath sections may be interconnected or bridged with impedance sheath bonding devices such as 25 if desired, as a further guarantee of continuity of the sheath circuit.

It will be of course apparent that other beginning or terminating bonding arrangements to be employed with the cross-bond arrangement in accordance with our invention may be devised including cases where it is desired to continue the bonding according to some other arrangement either by utilizing the connections shown in Figures 6 and 7, or in other ways not illustrated, since many variations are possible.

lVhile our invention has been thus far illustrated as applied to a three-phase system, it is obvious that the same principle and similar connections may be employed on metallic sheathed single-conductor cables for the transmission of single-phase, or polyphase currents in general, and it will be therefore understood that we do not limit ourselves to the application of the instant invention to a three-phase system.

In Figure 8 we have illustrated a single phase line, comprising two metallic sheathed single conductor cables, to which the principles of cross bonding as above disclosed have been applied. It is to be noted that in this figure the corresponding ends of each section of the sheath of one of the cables are grounded. In this figure the various parts similar to those previously described have been given similar refe-rence numerals.

It will be further evident that the ground connection may be otherwise made, for example, between the center' of the cable sheath lengths of the same phase in every section and the auxiliary ground cable or ground as illustrated in Figure 9. This location of the ground connection would be practical and desirable for cables buried directly in the earth. For such a case, the ground point in all the vector diagrams illustrated in Figures 2, B and 4 would be moved to the center of the base of each triangle if the ground connections were all made for example to the centers of sheath lengths on A-phase cable only. This vector diagram is illustrated in Figure l0. In this arrangement if we assume that the center of the A phase cable sheath is grounded then the maximum voltage to ground willoccur at the cross bond between the B and the C phase cable sheaths. The maximum voltage to ground is, however, appreciably lower than in the cases previously described, being only approximately 86% of the voltage induced along one section of cable sheath.

In any event, the feature of essential importance in our invention is that the sheath sections be continuously cross-bonded throughout and that only one similarly positioned point `of every consecutive sheath section (with occasional omissions where necessary) be grounded or connected to an auxiliary cable which is preferably grounded at one or more points.

While we have disclosed a preferred embodiment of our invention, it will be understood that we do not wish to be limited thereto. Changes may be made therein other than those above outlined without departing from the essence of the invention or the spirit and scope of the appended claims.

What we claim and desire to secure by Letters Patent is:

1. An electric circuit for transmitting alternating current comprising insulated conductors enclosed in metallic sheaths physically separated from each other, the sheaths comprising a plurality of sections serially electrically bonded to one another in a continuous transposed relation, and a grounding connection to one transposition bond at each transposition point.

2. An electric circuit for transmitting alternating current comprising insulated conductors enclosed in metallic sheaths physically separated from each other. the sheaths comprising a plurality of sections serially electrically bonded to one another in a continuous transposed relation, and one similarly connected transposition bond at each transposition point being grounded.

3. An electric circuit for transmitting poly-phase alternating currents comprising a plurality of insulated conductors, one for each phase, each enclosed in a metallic sheath physically separate from the other sheaths and divided into electrically discontinuous sections of approximately equal length. the adjoining sections of sheaths being ,serially connected to one another in a transposed relation` and a similarly positioned point of one cable sheath in each of several consecutive sections connected to ground.

4. An electric circuit for transmitting three-phase currents comprising three insulated conductors enclosed in metallic sheaths physically separated from each other and divided into insulated sections of approximately equal length with the sheath lengths serially connected to one another in a continuous transposed relation throughout, and one similarly connected transposition bond at each transposition point grounded t0 an auxiliary ground cable.

5. An electric circuit for transmitting three-phase currents comprising a line of three insulated conductors enclosed in metallic sheaths physically separated from each other, the sheath of each conductor being divided into electrically discontinuous sections of approximately equal length, the adjoining sections of .sheaths being serially connected to one another in a transposed relation, and one similarly positioned sheath end of each section also connected to ground at consecutive transposition points along the line.

6. An electric power line transmitting alternating current over a plurality of single conductor metallic sheathed cables wherein the .sheaths are divided into sections with adliacent sections of the same cable insulated from one another and wherein the sections are cross bonded to form separate sheath circuits each circuit including one section of each cable and wherein the respective sheath circuits are grounded, characterized by the fact that the ground connections for the respective circuits are displaced from one another by an amount substantially equal to one section length and are at electrically similar points.

7 An electric power line transmitting alternating current over a plurality of single conductor metallic sheathed cables wherein the sheaths are divided into sections with adjacent sections of the same cable insulated from one another and wherein the sections are cross bonded to form separate sheath circuits each circuit including one section of each cable and wherein the respective sheath circuits are grounded, characterized by the fact that the ground connections for the respective circuits are located at cross bonding points and displaced from one another by an amount substantially equal to one section length and are at electrically similar points.

8. A three phase electric power line, including three side by side metallic sheathed cables. one for each phase, each cable sheath comprising a plurality of sections insulated from one another, cross bonding between the sections producing three similar separate il' sheath circuits, ground connections to each sheath circuit at a plurality of points spaced three sections apart, characterized by the fact that the ground connections for the respective circuits are spaced from one another by the length of one section and are all at electrically similar points.

In witness whereof, we hereunto subscribe our names this 26 day of March, 1930.

KENNETH lV. MILLER. DENNEY W. ROPER.

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